1. Field of the Invention
This invention relates to electro-acoustical transducers and, more particularly, to a novel loudspeaker apparatus having a pair of diaphrams and having their rear sides connected by an elongated air passageway and being capable of generating improved acoustical wave energy distribution.
2. Description of the Prior Art
In the acoustic field a moving piston or "cone" almost universally used to generate lower register acoustic energy suffers from a drawback inherent in its mechanical construction. In motion, negative pressure is generated behind the cone as well as positive pressure in front of the cone. Unless suppressed in some manner this "back wave," as it is known, will cancel the desired "front wave," since it is equal and opposite in phase. This results in complete loss of lower audio tones and is completely unacceptable to a listener.
The simplest practical way to suppress the "back wave" is to enclose the loudspeaker in a rigid box. The stiffness of the enclosure is such that the pressures within the enclosure generated when the loudspeaker cone moves, are completely contained. This allows unhindered propagation of the front wave and all low register tones can be heard. This "enclosure" solution of the back wave problem is universally used today, in one of its several variations.
The stiffness of the air within the enclosure effectively reduces the compliance of the loudspeaker's moving member, which results in a very undesirable condition. Mechanically speaking, the mass of the moving element and its compliance result in a resonant condition. Electrically, this resonance results in a large rise in system impedance around the resonant frequency. This impedance rise can very easily reach 5 to 10 times the nominal value and results in very little power transfer from the driving source over the frequency range effected. Ideally, moving the resonance, due to moving mass and its concurrent stiffness, down to sub-sonic frequency is the answer to the problem. Physical limitations on the enclosure size and loudspeaker mechanical construction of the conventional equipment in use today render this condition essentially unattainable. System resonances almost invariably are within the audible spectrum.
Therefore, it is advantageous to remove the stiffening effect of the air within an enclosure, thus allowing the practical attainment of the very low resonant condition desired. Further, a means of utilizing the energy contained in the back wave, to re-inforce the front wave is desired.
In its simplest possible form a pair of back-to-back loudspeakers are so connected that their cones move simultaneously in the same direction. Volume change within the enclosure due to motion of the first cone is now zero since motion of the second one introduces a negative volume change exactly equal to the positive change due to the first cone. As is quite evident, this effect is only possible when the distance between cones is small compared to the wave length of the sound concerned. The effect of this zero volume change is zero pressure change within the enclosure, thus no stiffness is added to the mechanical system by the air within the enclosure. However, in this simplest form, cancellation effects due to the acoustic radiation of the pair of cones is still undesirable.
Prior art disclosures of this latter loudspeaker system are represented by U.S. Pat. Nos. 3,136,382 and 3,393,764.
Therefore, a need has long existed to provide an amplifying system including electro-acoustic transducers which will incorporate an infinite baffle approaching acoustical transparency whereby acoustical wave energy propagation and distribution are greatly improved.